It is known that dietary fibers have useful physiological effects when they are taken orally. For example, it is known that they have effects such as a laxative effect, normalization of cholesterol metabolism, improvement of sugar tolerance, promotion of insulin secretion, and elimination of toxic substances from the body.
On the other hand, it is also known that xylo-oligosaccharides and substances having an antioxidative activity have useful physiological effects Xylo-oligosaccharides reach the intestines without being digested by digestive enzymes of human, and have effects such as improvement of constipation symptoms and improvement of enteric environment, and have a characteristic that they exhibit a great effect on the improvement of enteric environment even at low doses. Further, it has been revealed that substances having an antioxidative activity express preferable effects such as antihypertension, elimination of active oxygen, suppression of peroxide production, suppression of rise in cholesterol, and promotion of fat metabolism.
Accordingly, food, etc., by which these effects can be safely obtained and which can be produced at low cost are demanded, and it is also demanded that food materials to be used as raw materials of such foods are provided at low prices. By applying the food materials to foods, it becomes possible to easily produce foods, etc., having these effects. In order to obtain these food materials at low prices, efficient production is desired.
Conventionally, the following method has been known as a method for producing food materials thus described: a method comprising the steps of conducting a fermentation treatment wherein a koji fungus is inoculated into and grown in a dietary fiber as a raw material derived from husks of cereals and beans, and then conducting a hydrolysis treatment by acids (for example, Patent Document 1). In addition, the following method has been known as a method for producing a dietary fiber which contains a xylo-oligosaccharide and which has an antioxidant activity: a method for producing an antioxidant dietary fiber comprising the steps of subjecting a plant fiber such as bagasse to a high pressure steam treatment and an explosion treatment, and then converting the treated fiber into sugar by an enzymatic treatment using xylanase and cellulase (for example, Patent Document 2).
Other than the methods mentioned above, the following method has been known as a method for producing a xylo-oligosaccharide having an effect on controlling intestinal functions, etc.; a method for obtaining a xylo-oligosaccharide comprising the steps of subjecting a chemical pulp as a raw material to an enzymatic treatment and hydrolyzing the treated pulp (for example, Patent Document 3). Further, the following methods have been disclosed: a method for obtaining a cereal grain distilled liquor containing a large amount of ferulic acid, which is an antioxidant substance, by using a koji fungus (for example, Patent Document 4), and a method for separating and obtaining ferulic acid, which is a substance having an antioxtdative activity, from a byproduct of liquors produced with the use of a koji fungus which produces citric acid (for example, Patent Document 5).    Patent Document 1: Japanese Laid-open Patent Application No. 06-7099    Patent Document 2: Japanese Laid-open Patent Application No. 2002-204674    Patent Document 3: Japanese Laid-open Patent Application No. 2003-48901    Patent Document 4: Japanese Laid-open Patent Application No 2004-236634    Patent Document 5: Japanese Laid-open Patent Application No. 2003-52351